A solar collector absorbs solar radiation and converts it into heat (photo-thermal conversion). The high efficiency solar collector implies maximum absorption of incident solar radiation with a minimum thermal and optical loss. Selective coating characteristics of the absorber and the vacuum insulation are the most important parts of an advanced collector. A selective surface has high absorption (low reflection) for the solar spectrum and low emissivity for the infrared heat radiation. The selective coating used in the collector consists of a thin, precise layer of an amorphous semiconductor plated on to a metal substratum; this has a low emission for long-wave radiation.